Liquid crystal display device operating in a vertically aligned mode comprising an optically biaxial retardation film

ABSTRACT

A liquid crystal display device includes a first substrate and a second substrate sandwiching a liquid crystal layer therebetween, a first polarizer disposed adjacent to the first substrate at a side opposite to a side of the first polarizer facing the liquid crystal layer, with a first gap between the first polarizer and the first substrate, a second polarizer disposed adjacent to the second substrate at a side opposite to a side of the second polarizer facing the liquid crystal layer, with a second gap between the second polarizer and the second substrate, wherein at least one of the first and second gaps includes therein a first retardation film having a positive optical anisotropy and a second retardation film having a negative optical anisotropy, such that the first retardation film is disposed closer to the liquid crystal layer with respect to the second retardation film.

This is a divisional of application Ser. No. 10/667,566, filed Sep. 22,2003, now U.S. Pat. No. 7,075,609, which is a divisional of applicationSer. No. 08/939,822, filed Sep. 29, 1997, now U.S. Pat. No. 6,642,981,issued Nov. 4, 2003.

BACKGROUND OF THE INVENTION

The present invention generally relates to liquid crystal displaydevices and more particularly to a liquid crystal display deviceoperating in a so-called VA (Vertically Aligned) mode in which liquidcrystal molecules having a negative dielectric anisotropy or positivedielectric anisotropy are aligned generally perpendicularly to a panelsurface of the liquid crystal display device.

Liquid crystal display devices are used as a display device of variousinformation processing apparatuses such as a computer. Liquid crystaldisplay devices, having a compact size and consuming little electricpower, are particularly suitable for application in portable informationprocessing apparatuses. On the other hand, use of such liquid crystaldisplay devices also in a fixed-type information processing apparatussuch as a desktop-type computer, is also studied.

Conventional liquid crystal display devices generally use a so-called TN(Twisted Nematic)-mode construction in which p-type liquid crystalmolecules having a positive dielectric anisotropy are alignedhorizontally between a pair of mutually opposing panel substrates,wherein the liquid crystal molecules adjacent to one panel substrate andthe liquid crystal molecules adjacent to the other panel substrate arealigned in respective directions crossing with each otherperpendicularly.

In such a TN-mode liquid crystal display device, various liquid crystalsare already developed, and the liquid crystal display device can befabricated by a well-established process with low cost.

On the other hand, a TN-mode liquid crystal display device has adrawback in realizing a high contrast representation of images. Itshould be noted that a TN-mode liquid crystal display device provides ablack representation by causing the liquid crystal molecules to alignvertically to the principal surface of the panel substrate by applying adriving electric field, while the liquid crystal molecules immediatelyadjacent to the panel substrate tend to maintain the horizontalalignment even when the driving electric field is applied. Thereby, thebirefringence associated with such horizontal liquid crystal moleculesallows a passage of light even in the activated state in which thepassage of light through the liquid crystal layer should be interruptedcompletely. Thus, there occurs a leakage of light or coloring of thepanel when an attempt is made in a TN-mode liquid crystal display deviceto display a white image on a black background (so-called “normallyblack mode”) as is commonly adopted in a CRT display device. Thus, theblack representation becomes worse than that of a “normally white mode,”in which black images are displayed on a white background, because ofdispersion. This is the reason why conventional TN-mode liquid crystaldisplay devices are operated in the normally white mode.

A VA-mode liquid crystal display device is a liquid crystal displaydevice in which liquid crystal molecules having a negative or positivedielectric anisotropy are confined between a pair of panel substrates ina state that the liquid crystal molecules are aligned in a directiongenerally perpendicular to the principal surface of the panel substratesin a non-activated state of the liquid crystal display device. Thus, alight passes through a liquid crystal layer in such a liquid crystaldisplay device without changing the polarization plane thereof in thenon-activated state of the liquid crystal device, and the light iseffectively interrupted by a pair of polarizers disposed at both sidesof the liquid crystal layer in a crossed Nicol state. In such a VA-modeliquid crystal display device, therefore, it is possible to achieve anear-ideal black representation in the non-activated state of the liquidcrystal display device. In other words, such a VA-mode liquid crystaldisplay device can easily achieve a very high contrast representationnot possible by a TN-mode liquid crystal display device.

In an activated state of a VA-mode liquid crystal display device, itshould be noted that the liquid crystal molecules are aligned generallyparallel to the panel substrates due to the electric field applied tothe liquid crystal molecules, and a rotation is induced in thepolarization state of an incident optical beam. Thereby, the liquidcrystal molecules thus activated show a 90°-twist between the firstpanel substrate and the second panel substrate.

The VA-mode itself has been known for a long time. Further, there existsa report about the property of a liquid crystal having a negativedielectric anisotropy (D. de Rossi, J. Appl. Phys. 49(3), March, 1978).

On the other hand, it has been thought conventionally that a VA-modeliquid crystal display device cannot provide the quality ofrepresentation comparative to that of a TN-mode liquid crystal displaydevice, in terms of response time, viewing-angle characteristics,voltage retention (or voltage holding ratio), and the like. Thus, littleeffort has been made so far for realizing a practical liquid crystaldisplay device using a VA-mode liquid crystal. Particularly, it has beenbelieved that construction of an active-matrix liquid crystal displaydevice that uses thin-film transistors (TFT) is very difficult.

As a VA-mode liquid crystal can provide a contrast ratio superior tothat of a conventional CRT (cathode-ray tube) display device, it ispredicted that the major target of such a VA-mode liquid crystal displaydevice would be to replace conventional CRT display devices. In order toachieve this target, however, it is particularly necessary to improvethe viewing-angle characteristics of the display device, in addition tousual requirements of increasing the display area and improving theresponse.

Japanese Laid-open Patent Publication 62-180326 describes a VA-modeliquid crystal display device in which a liquid crystal layer formed ofliquid crystal molecules having a negative dielectric anisotropy, isconfined between a pair of glass substrates such that the liquid crystalmolecules align generally perpendicularly to the substrate surface in anon-activated state thereof in which no drive voltage is applied acrossthe glass substrates. The reference further describes a construction tocause a 90°-twist for the liquid crystal molecules in the directiongenerally parallel to the substrate surface in the activated statethereof in which the drive voltage is applied across the substrates.Further, the reference teaches to dispose a polarizer and an analyzer atrespective outer sides of the glass substrates such that respectiveoptical absorption axes intersect perpendicularly with each other.

Japanese Laid-open Patent Publication 3-5721, on the other hand,describes a VA-mode liquid crystal display device in which a liquidcrystal layer formed of liquid crystal molecules having a negativedielectric anisotropy, is confined between a pair of substrates, In theabove noted reference, the liquid crystal layer has a retardation set ina range between 0.6 μm and 0.9 μm, and first and second birefringencemedia are disposed at both sides of a liquid crystal panel thus formed.Further, the reference teaches to provide a polarizer and an analyzer atrespective outer sides of the foregoing birefringence media so as tocross the respective optical absorption axes perpendicularly. Further,the reference teaches to set the optical absorption axes so as to form a45° angle with respect to the optical axes of the birefringence media.

Further, Japanese Laid-open Patent Publication 5-113561 describes aphoto-conduction type liquid crystal light valve, wherein the referenceteaches the use of a liquid crystal of negative dielectric anisotropyfor a liquid crystal layer provided adjacent to a photo-conductionlayer, such that the liquid crystal molecules align generallyperpendicularly to the electrode surface in the non-activated state ofthe liquid crystal layer. Further, the reference teaches a feature toset the retardation of the liquid crystal layer to be 0.3 μm or more.

Further, Japanese Laid-open Patent Publication 5-113561 describes aVA-mode liquid crystal display device that includes optical compensationmeans having a negative optical activity in addition to a pair ofsubstrates that confine a liquid crystal layer of liquid crystalmolecules having a negative dielectric anisotropy therebetween, whereinthe liquid crystal display device further includes first and secondquarter-wavelength phase shift plates such that the first phase shiftplate has a positive optical activity and an optical axis parallel tothe substrates and such that the second phase shift plate has a negativeoptical activity and an optical axis parallel to the optical axis of thefirst phase shift plate. The liquid crystal display device of thereference further includes a polarizer and an analyzer in a crossedNicol state such that the polarizer and the analyzer sandwich theforegoing construction therebetween.

However, such conventional VA-mode liquid crystal devices, while capableof providing a contrast ratio exceeding the contrast ratio achieved bythe conventional TN-mode or STN-mode (supertwisted nematic mode) liquidcrystal display devices, cannot provide response, viewing-anglecharacteristics, brightness and colorless representation required for adesktop display device.

SUMMARY OF THE INVENTION

Accordingly, it is a general object of the present invention to providea novel and useful liquid crystal display device wherein the foregoingproblems are eliminated.

Another and more specific object of the present invention is to providea VA-mode liquid crystal display device that uses a liquid crystalhaving a negative or positive dielectric anisotropy, in which the liquidcrystal display device is optimized with respect to response,viewing-angle and contrast of representation.

Another object of the present invention is to provide a liquid crystaldisplay device, comprising:

a first substrate and a second substrate sandwiching a liquid crystallayer therebetween;

a first polarizer disposed adjacent to said first substrate at a sideopposite to a side of said first polarizer facing said liquid crystallayer, with a first gap between said first polarizer and said firstsubstrate;

a second polarizer disposed adjacent to said second substrate at a sideopposite to a side of said second polarizer facing said liquid crystallayer, with a second gap between said second polarizer and said secondsubstrate;

at least one of said first and second gaps including therein a firstretardation film having a positive optical anisotropy and a secondretardation film having a negative optical anisotropy, such that saidfirst retardation film is disposed closer to said liquid crystal layerwith respect to said second retardation film.

According to the present invention, a wide viewing-angle is realized ina VA-mode liquid crystal display device by disposing the first andsecond retardation films adjacent to the liquid crystal layer.

Another object of the present invention is to provide a liquid crystaldisplay device, comprising:

a first substrate and a second substrate sandwiching a liquid crystallayer therebetween;

a first polarizer disposed adjacent to said first substrate at a sideopposite to a side of said first polarizer facing said liquid crystallayer, with a first gap between said first polarizer and said firstsubstrate;

a second polarizer disposed adjacent to said second substrate at a sideopposite to a side of said second polarizer facing said liquid crystallayer, with a second gap between said second polarizer and said secondsubstrate;

at least one of said first and second gaps including therein anoptically biaxial retardation film.

According to the present invention, a wide viewing-angle can be realizedby using the optically biaxial retardation film adjacent to the liquidcrystal layer.

Another object of the present invention is to provide a liquid crystaldisplay device, comprising:

first and second substrates disposed substantially parallel to eachother, said first substrate having a first principal surface at a sidethereof facing said second substrate, said second substrate having asecond principal surface at a side thereof facing said first substrate;

a first electrode pattern provided on said first principal surface ofsaid first substrate;

a second electrode pattern provided on said second principal surface ofsaid second substrate;

a first molecular orientation film disposed on said first principalsurface of said first substrate so as to cover said first electrodepattern;

a second molecular orientation film disposed on said second principalsurface of said second substrate so as to cover said second electrodepattern;

a liquid crystal layer confined between said first and second molecularorientation films;

said liquid crystal layer containing liquid molecules such that a majoraxis of said liquid crystal molecule aligns generally perpendicularly toat least one of said first and second principal surfaces;

said liquid crystal layer having a retardation of about 80 nm or morebut below about 400 nm.

According to the present invention, it becomes possible to construct theliquid crystal display device to have a wide viewing-angle, highresponse speed and a colorless, high-contrast representation.

Other objects and further features of the present invention will becomeapparent from the following detailed description when read inconjunction with the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the fundamental construction of a liquidcrystal display device of the present invention;

FIGS. 2A and 2B are diagrams respectively showing the relationshipbetween the contrast and orientation of polarizers used in the liquidcrystal display device of FIG. 1 and the definition of parameters usedin FIG. 2A;

FIGS. 3A-3D are diagrams showing a dynamic performance of the liquidcrystal display device of FIG. 1 for various constructions;

FIGS. 4A and 4B are diagrams showing the principle of a VA-mode liquidcrystal display device that uses a liquid crystal having a negativedielectric anisotropy;

FIGS. 5A and 5B are diagrams showing the principle of a VA-mode liquidcrystal display device that uses a liquid crystal having a positivedielectric anisotropy;

FIG. 6A is a diagram showing a modification of the liquid crystaldisplay device of FIG. 1 in which a retardation film is added adjacentto a liquid crystal panel in the construction of FIG. 1;

FIG. 6B is a diagram showing a definition of azimuth angle and polarangle;

FIGS. 7-16 are diagrams showing the viewing-angle characteristics of theliquid crystal display device of FIG. 6A for various settings ofretardation of the retardation film;

FIGS. 17-22 are diagrams showing the viewing-angle characteristics ofthe liquid crystal display device of FIG. 6A for various thicknesses dof the liquid crystal layer in the liquid crystal panel;

FIGS. 23-28 are diagrams showing a transmittance of the liquid crystaldisplay device of FIG. 6A for various thicknesses d of the liquidcrystal layer in the liquid crystal panel;

FIGS. 29-33 are diagrams showing a coloring of the liquid crystaldisplay device of FIG. 6A for various thicknesses d of the liquidcrystal layer;

FIGS. 34-36 are diagrams showing the viewing-angle characteristics ofthe liquid crystal display device of FIG. 6A for various settings of thetwist angle of the liquid crystal molecules forming the liquid crystallayer in the device of FIG. 6A;

FIG. 37 is a diagram showing the black-mode transmittance of the liquidcrystal display device of FIG. 6A;

FIGS. 38A and 38B are diagrams showing the orientation of the liquidcrystal molecules in the liquid crystal layer of the liquid crystaldisplay device of FIG. 6A for the case in which a chiral substance isadded to the liquid crystal layer;

FIGS. 39A and 39B are diagrams showing the orientation of the liquidcrystal molecules of the liquid crystal display device of FIG. 6A forthe case in which no chiral substance is added to the liquid crystallayer;

FIG. 40 is a diagram showing the viewing-angle characteristics of theliquid crystal display device of FIG. 6A for the case in which a chiralsubstance is added to the liquid crystal layer;

FIG. 41 is a diagram showing a transmittance of the liquid crystaldisplay device of FIG. 6A for the case in which a chiral substance isadded to the liquid crystal layer;

FIG. 42 is a diagram showing a transmittance of the liquid crystaldisplay device of FIG. 6A for the case in which no chiral substance isadded to the liquid crystal layer;

FIGS. 43-46 are diagrams showing the viewing-angle characteristics ofthe liquid crystal display device of FIG. 6A for various pretilt anglesof the liquid crystal molecules;

FIG. 47 is a diagram showing the viewing-angle characteristics of atypical twist-nematic liquid crystal display device;

FIG. 48 is a diagram showing a construction of a liquid crystal displaydevice according to a first embodiment of the present invention;

FIGS. 49A and 49B are diagrams showing the viewing-angle characteristicsof the liquid crystal display device of FIG. 48;

FIGS. 50A and 50B are diagrams showing the viewing-angle characteristicsof the liquid crystal display device of FIG. 48 for a case in which aretardation film is added;

FIG. 51 is a diagram showing the viewing-angle characteristics of theliquid crystal display device of FIG. 48 for a case in which a pair ofretardation films are added and the pretilt angle of the liquid crystalmolecules is set to 75°;

FIGS. 52 and 53 are diagrams showing a response of the liquid crystaldisplay device according to a second embodiment of the presentinvention;

FIG. 54 is a diagram showing a construction of the liquid crystaldisplay device according to a third embodiment of the present invention;

FIG. 55 is a diagram showing a black-mode transmittance of the liquidcrystal display device of FIG. 54;

FIG. 56 is another diagram showing a black-mode transmittance of theliquid crystal display device of FIG. 54;

FIG. 57 is a diagram showing the viewing-angle characteristics of theliquid crystal display device of FIG. 54;

FIG. 58 is a diagram showing the viewing-angle characteristics of theliquid crystal display device of FIG. 54 for a case in which the orderof positive and negative retardation films in the construction of FIG.54 is reversed;

FIG. 59 is a diagram showing the viewing-angle characteristics of theliquid crystal display device of FIG. 54 in which the retardationcompensation film is omitted;

FIG. 60 is a diagram showing the construction of a liquid crystaldisplay device according to a fourth embodiment of the presentinvention;

FIG. 61 is another diagram showing a black-mode transmittance of theliquid crystal display device of FIG. 60;

FIG. 62 is a further diagram showing a black-mode transmittance of theliquid crystal display device of FIG. 60;

FIG. 63 is a diagram showing viewing-angle characteristics of the liquidcrystal display device of FIG. 60;

FIG. 64 is a diagram showing the construction of a liquid crystaldisplay device according to a fifth embodiment of the present invention;

FIG. 65 is a diagram showing viewing-angle characteristics of the liquidcrystal display device of FIG. 64;

FIG. 66 is a diagram showing a construction of the liquid crystaldisplay device according to a sixth embodiment of the present invention;

FIG. 67 is a diagram showing the black-mode transmittance of the liquidcrystal display device of FIG. 66;

FIG. 68 is another diagram showing the black-mode transmittance of theliquid crystal display device of FIG. 66;

FIG. 69 is a diagram showing the viewing-angle characteristics of theliquid crystal display device of FIG. 66;

FIG. 70 is a diagram showing the construction of a liquid crystaldisplay device according to a seventh embodiment of the presentinvention;

FIG. 71 is a diagram showing the viewing-angle characteristics of theliquid crystal display device of FIG. 70;

FIG. 72 is a diagram showing the construction of a liquid crystaldisplay device according to an eighth embodiment of the presentinvention;

FIG. 73 is a diagram showing the black-mode transmittance of the liquidcrystal display device of FIG. 72;

FIG. 74 is a diagram showing the black-mode transmittance of the liquidcrystal display device of FIG. 72;

FIG. 75 is a diagram showing the viewing-angle characteristics of theliquid crystal display device of FIG. 72;

FIG. 76 is a diagram showing the construction of a liquid crystaldisplay device according to a ninth embodiment of the present invention;

FIG. 77 is a diagram showing the viewing-angle characteristics of theliquid crystal display device of FIG. 76;

FIG. 78 is a diagram showing the construction of a liquid crystaldisplay device according to a tenth embodiment of the present invention;

FIG. 79 is a diagram showing the viewing-angle characteristics of theliquid crystal display device of FIG. 78;

FIG. 80 is a diagram showing a construction of the liquid crystaldisplay device according to an eleventh embodiment of the presentinvention;

FIG. 81 is a diagram showing viewing-angle characteristics of the liquidcrystal display device of FIG. 80;

FIGS. 82A-82C are diagrams showing a domain structure of the liquidcrystal display device of any of the preceding embodiments;

FIGS. 83A-83C are diagrams showing a domain structure of the liquidcrystal display device according to a twelfth embodiment of the presentinvention;

FIGS. 84A-84C are diagrams showing a domain structure of the liquidcrystal display device according to a modification of the twelfthembodiment;

FIG. 85 is a diagram showing viewing-angle characteristics of the liquidcrystal display device of the twelfth embodiment;

FIG. 86 is a diagram showing the result of simulation for theviewing-angle characteristics of the liquid crystal display device ofthe twelfth embodiment;

FIG. 87 is a diagram showing a construction of a direct-view type liquidcrystal display device that uses the vertically aligned liquid crystaldisplay device of the present invention:

FIG. 88 is a diagram showing the construction of a liquid crystaldisplay device according to a thirteenth embodiment of the presentinvention;

FIG. 89 is a diagram showing the black-mode transmittance of the liquidcrystal display device of FIG. 88;

FIG. 90 is a diagram showing the polar-angle dependence of theblack-mode transmittance for various structures of the thirteenthembodiment;

FIG. 91A-91D show various structures used in the experiment of FIG. 90;

FIGS. 92A and 92B show the viewing-angle characteristics of the liquidcrystal display device of FIG. 88 in comparison with a case in whichretardation films are eliminated;

FIG. 93 is a diagram showing the construction of a liquid crystaldisplay device according to a fourteenth embodiment of the presentinvention; and

FIG. 94 is diagram showing the viewing-angle characteristics of theliquid crystal display device of FIG. 93.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[Principle]

First, the principle of the present invention will be explained.

FIG. 1 shows the construction of a liquid crystal display device 10,wherein it should be noted that FIG. 1 represents the fundamentalconstruction of the liquid crystal display device of the presentinvention.

Referring to FIG. 1, the liquid crystal device 10 includes a pair ofmutually opposing glass substrates 11A and 11B confining therebetween aliquid crystal layer 12 having a thickness d. Thereby, the substrates11A and 11B and the liquid crystal layer 12 form together a liquidcrystal panel 11. Further, a first polarizer 13A having an absorptionaxis 13 a in a first direction is disposed below the liquid crystalpanel 11, and a second polarizer (called also “analyzer”) 13B having anabsorption axis 13 b in a second direction is disposed above the liquidcrystal panel 11.

In the liquid crystal display device 10 of FIG. 1, it should be notedthat the liquid crystal layer 12 is formed of an n-type liquid crystalhaving a negative dielectric anisotropy or a p-type liquid crystalhaving a positive dielectric anisotropy. Thereby, each of the substrates11A and 11B carries thereon a molecular alignment layer (not shown), andthe molecular alignment films thus formed are prepared, typically bymeans of rubbing, such that liquid crystal molecules 12 a adjacent tothe lower substrate 11A are aligned generally vertically to thesubstrate 11A. Similarly, liquid crystal molecules 12 b adjacent to theupper substrate 11B are aligned generally vertically to the substrate11B. In other words, the liquid crystal display device 10 is a device ofthe so-called VA (vertically aligned)-mode.

In the construction of FIG. 1, it should be noted that the lowersubstrate 11A carries, on an upper major surface thereof, a firstmolecular alignment layer (not illustrated, to be described later withreference to embodiments), wherein the first molecular alignment layeris subjected to a rubbing process in a direction offset in thecounter-clockwise direction from the longer edge of the substrate 11A byan angle of 22.5°. The first molecular alignment layer thus processedcauses a director, which indicates the direction of alignment of theliquid crystal molecules 12 a, to point a direction offset in an upwarddirection from the rubbing direction of the first molecular alignmentlayer by an angle of about 87°.

Similarly, the upper substrate 11B carries, on a lower major surfacethereof, a second molecular alignment layer (not illustrated, to bedescribed later with reference to embodiments), wherein the secondmolecular alignment layer is subjected to a rubbing process in adirection offset in the clockwise direction from the longer edge of thesubstrate 11B by an angle of 22.5°. The second molecular alignment layerthus processed causes a director of the liquid crystal molecules 12 b topoint a direction offset in a downward direction from the rubbingdirection of the second molecular alignment layer by an angle of about87°. Thereby, the liquid crystal molecules in the liquid crystal layer12 form a twist angle of 45° between the upper and lower substrates 11Aand 11B.

It should be noted that the substrates 11A and 11B are set in therespective orientations, when forming the liquid crystal panel 11, suchthat the general rubbing direction of the substrate 11A and the generalrubbing direction of the substrate 11B are opposite of each other.

As already noted, the polarizer 13A having the absorption axis 13 a isdisposed below the liquid crystal panel 11, wherein the polarizer 13Apolarizes an optical beam incident to the liquid crystal panel 11 fromthe lower direction, such that the plane of polarization isperpendicular to the foregoing absorption axis 13 a. Similarly, thepolarizer 13B having the absorption axis 13 b is disposed above theliquid crystal panel 11, wherein the polarizer 13B polarizes the opticalbeam incident to the liquid crystal panel 11 from the lower direction,such that the plane of polarization is perpendicular to the absorptionaxis 13 b.

Thus, by disposing the polarizers 13A and 13B such that respectiveabsorption axes 13 a and 13 b intersect each other perpendicularly, itis possible to interrupt the optical beam passing through the liquidcrystal panel 11 without a substantial change of the polarization plane.In other words, the polarizer 13B interrupts the optical beam polarizedby the polarizer 13A and passed through the liquid crystal panel 11without experiencing a rotation of the polarization plane, and theliquid crystal display device provides a black representation.

It should be noted that each of the substrates 11A and 11B carries atransparent electrode (not illustrated) in the state that thetransparent electrode is embedded in the molecular alignment layer. Inthe non-activated state of the liquid crystal device in which no drivevoltage is applied across the electrodes, the liquid crystal moleculesin the liquid crystal layer 12 align generally vertically to thesubstrates as in the case of the liquid crystal molecule 12 a or theliquid crystal molecule 12 b. Thereby, a near-ideal black representationis achieved in the non-activated state of the liquid crystal displaydevice.

In an activated state, on the other hand, the liquid crystal moleculesare aligned generally parallel to the substrates 11A and 11B. Thereby,the optical beam passing through the liquid crystal panel 11 experiencesa rotation of the polarization plane due to such horizontally alignedliquid crystal molecules and passes through the polarizer 13B. Thereby,the liquid crystal display device 10 provides a white representation inthe activated state.

FIG. 2A shows the contrast ratio achieved by the liquid crystal displaydevice 10 for the case in which the angle φ of the absorption axis 13 aand the angle θ of the absorption axis 13 b are changed variously,wherein the definition of the angles φ and θ is given in FIG. 2B. Thecontrast ratio was measured by comparing the transmittance of the liquidcrystal display device 10 for the non-activated state in which no drivevoltage is applied and the transmittance of an activated state in whicha drive voltage of 5 V is applied.

In the example of FIG. 2A, a liquid crystal supplied from E. Merck, Inc.(MJ95785, Δn=0.0813, Δ∈=−4.6) is used for the liquid crystal layer 12,wherein Δ∈ represents the dielectric anisotropy of the liquid crystal,while Δn represents the birefringence of the liquid crystal defined asΔn=n_(e)−n_(o), in which n_(e) is a refractive index of an extraordinaryray in the liquid crystal while n_(o) represents a refractive index ofan ordinary ray also in the liquid crystal. Further, a commerciallyavailable product of Nitto Denko KK (G1220DU) is used for the polarizers13A and 13B. The thickness d of the liquid crystal layer 12 is set to3.5 μm.

In FIG. 2B showing the definition of the angles φ and θ, it should benoted that, in order to represent the twist angle and to define thecenter of the twist clearly, the upper substrate 11B is illustrated in astate rotated by 180° to the state of FIG. 1.

Referring to FIG. 2A, it should be noted that the contrast ratio of theliquid crystal display device 10 becomes maximum in the crossed-Nickolstate in which the absorption axis 13 a of the polarizer 13A and theabsorption axis 13 b of the polarizer 13B intersect perpendicularly, andparticularly when the angle φ is set to 45° (φ=45°). In this state, itshould be noted that the absorption axis 13 a of the polarizer 13A formsan angle of 45° with respect to the center line C of twist, whichcoincides with a line represented in FIG. 2B by 0°-180°. In the crossedNickol state, therefore, the angle of the absorption axis 13 b of thepolarizer 13B with respect to the center line C of the twist becomes135°.

It will be obvious that a similar maximum of the contrast ratio is alsoachieved when the angles φ and θ are set respectively to −45° and −135°.In this case, the absorption axis 13 a of the polarizer 13A forms theangle of 135° with respect to the center line C of the twist, while theabsorption axis 13 b of the polarizer 13B forms the angle of 45°.

As will be seen from FIG. 2A, the liquid crystal display device 10achieves a contrast ratio exceeding 700 for any settings of the angles φand θ. This is a remarkable improvement over normal TN-mode liquidcrystal display devices, in which the maximum contrast ratio is in theorder of 100 at best.

FIGS. 3A-3D show the operational characteristics of the liquid crystaldisplay device 10 of FIG. 1, wherein the results shown in FIGS. 3A-3Dare for the liquid crystal display device having the constructiondescribed already.

Referring to the drawings, FIG. 3A shows the waveform of the drivevoltage pulse applied to the liquid crystal layer in the liquid crystalpanel 11, while FIG. 3B shows the change of the transmittance occurringin the liquid crystal panel 11 in response to the drive voltage pulse ofFIG. 3A.

In FIG. 3B, the continuous line represents the result in which no chiralsubstance is added to the liquid crystal layer 12 in the panel 11, whilethe broken line represents the result in which a chiral substance isadded, as is commonly practiced in a TN-mode liquid crystal displaydevice. The result of FIG. 3B is for the case in which the thickness dof the liquid crystal layer 12 is set to 3.5 μm and the twist angle ofthe liquid crystal molecules is set to 45° as already noted. In theexample of FIG. 3B, the chiral substance admixed in the liquid crystallayer 12 has a pitch p set such that a ratio d/p with respect to thethickness d of the liquid crystal layer is 0.25.

The result of FIG. 3B clearly indicates that admixing of the chiralsubstance in the liquid crystal layer 12 provides an adverse effect onthe dynamic response of the liquid crystal display device 10substantially. More specifically, it is noted that, while the liquidcrystal display device 10 shows a high optical transmittancecontinuously in response to the drive voltage pulse of FIG. 3A for theentire duration of the drive voltage pulse when no chiral substance isadded to the liquid crystal layer, the optical transmittance decreaseswith time when the chiral substance is added, even when the electricdrive pulse is applied to the liquid crystal layer 12 continuously witha constant magnitude.

FIG. 3C shows the transmittance of the liquid crystal display device 10in response to the voltage pulse of FIG. 3A for the case in which thethickness d of the liquid crystal layer 12 is set to 3.5 μm, wherein thetwist angle of the liquid crystal molecules is changed in the experimentof FIG. 3C in the range between 0°-90°. As will be seen clearly fromFIG. 3C, the dynamic response is not affected substantially by the twistangle of the liquid crystal molecules. In the experiment of FIG. 3C, itshould be noted that the twist angle was controlled by setting therubbing directions of the substrates 11A and 11B.

FIG. 3D shows the change of the transmittance in response to the voltagepulse of FIG. 3A of the liquid crystal display device 10 wherein thethickness d of the liquid crystal layer 12 is changed variously in therange between 4.5 μm and 2.5 μm. As can be seen clearly from FIG. 3D,the magnitude of change of the transmittance decreases with the decreaseof the thickness d. Further, it should be noted that a turn-on transienttime T_(on), indicating the time needed for the transmittance of theliquid crystal display device 10 to reach, starting from a 0%transmittance state, a 90% transmittance state of the saturatedtransmittance (T=100%), decreases with decreasing thickness d of theliquid crystal layer 12. Similarly, a turn-off transient time T_(off),indicating the time needed for the transmittance of the device 10 toreach, starting from a saturated transmittance state (T=100%), a 10%transmittance state of the saturated state, decreases with decreasingthickness d of the liquid crystal layer 12. In other words, the responseof the liquid crystal display device 10 becomes faster with decreasingthickness d of the liquid crystal layer 12. Particularly, the rising andfalling of the transmittance becomes very sharp when the thickness d isset to 2.5 μm or less.

FIGS. 4A and 4B show the operation of the liquid crystal display deviceof FIG. 1 for the case in which a liquid crystal having a negativedielectric anisotropy is used for the liquid crystal layer 12.

Referring to FIGS. 4A and 4B, it should be noted that the glasssubstrate 11A carries thereon an electrode pattern 11 a and a molecularalignment film 11 a′ while the glass substrate 11B carries thereon anelectrode pattern 11 b and a molecular alignment film 11 b.′ Further, aliquid crystal layer 12 is confined between the molecular alignmentfilms 11 a′ and 11 b.′

In the state of FIG. 4A, showing an non-activated state in which nodrive voltage is applied across the electrode patterns 11 a and 11 b, itshould be noted that the liquid crystal molecules align generallyperpendicularly to the principal surface of the substrate 11A or 11B asa result of the interaction with the molecular alignment film 11 a′ or11 b.′

When a drive voltage is applied across the electrode patterns 11 a and11 b, on the other hand, the liquid crystal molecules having thenegative dielectric anisotropy are aligned in a generally horizontaldirection such that the liquid crystal molecules intersect generallyperpendicularly to the driving electric field across the electrodepatterns 11 a and 11 b.

FIGS. 5A and 5B show the operation of the VA-mode liquid crystal displaydevice of FIG. 1 in which a liquid crystal having a positive dielectricanisotropy is used for the liquid crystal layer 12. In FIGS. 5A and 5B,those parts corresponding to the parts described previously aredesignated by the same reference numerals and the description thereofwill be omitted.

Referring to FIGS. 5A and 5B, it should be noted that no electrodepattern is formed on the substrate 11B, and a pair of adjacent electrodepatterns 11 a are formed on the substrate 11A.

In the non-activated state of FIG. 5A, the liquid crystal molecules arealigned generally vertically to the principal surface of the substrate11A or 11B, similarly to the case of FIG. 4A, while it should be notedthat the liquid crystal molecules are generally aligned horizontally inthe activated state of FIG. 5B, in which a drive voltage is appliedacross the adjacent pair of the electrode patterns 11 a, along theelectric field formed between the foregoing electrode patterns 11 a.

FIG. 6A shows the construction of a liquid crystal display device 20 inwhich a retardation film 14A is added to the structure of FIG. 1 belowthe liquid crystal panel 11 for improving the viewing-anglecharacteristics of the liquid crystal display device further. It shouldbe noted that the retardation film 14A compensates for a phase shift ofthe optical beam passing through or passed through the liquid crystallayer 12 in the liquid crystal panel 11.

In the construction of FIG. 6A, it should be noted that the retardationfilm 14A provides a negative retardation Δn·d₁ in the z-direction(Δn=n_(y)−n_(z)=n_(x)−n_(z); where n_(x), n_(y) and n_(z) representrefractive indices specified by a refractive index ellipsoidrespectively on the principal axes x, y and z; d₁ represents thethickness of the retardation film), wherein the retardation film 14A isdisposed between the polarizer 13A and the liquid crystal panel 11.Thereby, the retardation film 14A compensates for the birefringenceoccurring in the optical beam passing through the liquid crystal panel11.

FIGS. 7-16 represent the viewing-angle characteristics of the liquidcrystal display device 20 including the retardation film 14A, forvarious values of the retardation R′ produced by the retardation film14A, wherein each of FIGS. 5-14 shows a contrast ratio CR achieved bythe liquid crystal display device 20 in the form of contour lines. Inthe illustrated examples, the contrast ratio CR is represented for thevalues of 500.0, 200.0, 100.0, 50.0 and 10.0, wherein the contour linesare represented in a coordinate system shown in FIG. 4B specified by anazimuth angle and a polar angle. As indicated in FIG. 6B, the azimuthangle is measured in the plane parallel to the liquid crystal panel fromthe center line C of the twist, while the polar angle is measured from anormal to the liquid crystal panel. The polar angle becomes zero in thedirection perpendicular to the liquid crystal panel 11.

Each of FIGS. 7-16 includes the azimuth angles of 0.0°, 90.0°, 180.0°and 270.0° as represented along the circumference and a polar angle of0.0° to 80.0° in the form of concentric circles. In each of FIGS. 7-16,the center of the circle indicates the front direction of the liquidcrystal display device 20 where the polar angle is 0.0°. Further, theoutermost circle represents the polar angle of 80.0°. In the experimentsof FIGS. 7-16, the birefringence Δn of the liquid crystal panel is setto 0.0804, the thickness d to 3 μm, the twist angle of the liquidcrystal molecules to 45°, and the pretilt angle to 89°. Thus, the liquidcrystal panel 11 provides a retardation Δn·d of 241 nm.

In the example of FIG. 7, the retardation R′ is set to 108 nm. Thus, aratio R′/Δn·d indicating the ratio of the retardation R′ to theretardation of the liquid crystal panel 11 takes a value of 0.45. In theexample of FIG. 8, on the other hand, the retardation R′ is 144 nm andthe ratio R′/Δn·d takes a value of 0.6. Further, in the example of FIG.9, the retardation R′ is 180 nm and the ratio R′/Δn·d takes a value of0.75. In the example of FIG. 10, the retardation R′ is 198 nm and theratio R′/Δn·d takes a value of 0.82. In the example of FIG. 11, theretardation R′ is 216 nm and the ratio R′/Δn·d takes a value of 0.90. Inthe example of FIG. 12, the retardation R′ is 234 nm and the ratioR′/Δn·d takes a value of 0.97. In the example of FIG. 13, theretardation R′ is 252 nm and the ratio R′/Δn·d takes a value of 1.05. Inthe example of FIG. 14, the retardation R′ is 270 nm and the ratioR′/Δn·d takes a value of 1.12. In the example of FIG. 15, theretardation R′ is 288 nm and the ratio R′/Δn·d takes a value of 1.20.Further, in the example of FIG. 16, the retardation R′ is 324 nm and theratio R′/Δn·d takes a value of 1.34.

Referring to FIGS. 7-16, it should be noted that the liquid crystaldisplay device 20 provides particularly excellent viewing-anglecharacteristics in the condition of FIG. 11 or FIG. 12 in which theforegoing ratio R′/Δn·d is set near 1 (0.97 to 1.05). In other words,the result of FIGS. 7-16 clearly indicates that the viewing-anglecharacteristics of the liquid crystal display device 20 are improvedsubstantially by disposing the retardation film 14A adjacent to theliquid crystal panel 11 such that the total retardation of theretardation film(s) is generally equal to the retardation of the liquidcrystal panel.

It should be noted that the foregoing relationship holds also whenanother retardation film 14B is disposed above the liquid crystal panel11. In this case, the foregoing value R′ of the retardation is given asa sum of the retardation film 14A and the retardation film 14B.

FIGS. 17-22 show the viewing-angle characteristics of the liquid crystaldisplay device 20 of FIG. 6A for the case in which the thickness d ofthe liquid crystal layer 12 forming the liquid crystal panel 11 ischanged variously, while maintaining the total retardation R′ of theretardation films 14A and 14B to be generally equal to the retardationΔn·d of the liquid crystal panel 11. In FIGS. 17-22, it should be notedthat the contour designated by “CR=10” indicates the viewing-anglecharacteristics in which a contrast ratio of 10 are achieved. The sameapplies also to FIGS. 7-16 described previously.

Referring to FIGS. 17-22, it should be noted that the viewing-anglecharacteristics of the liquid crystal display device 20 are obviouslydeteriorated when the thickness d is reduced below 1 μm and hence theretardation Δn·d of the liquid crystal panel 11 is reduced below 82 nm.Further, when the thickness d exceeds 5 μm and the retardation Δn·dexceeds 410 nm, the viewing-angle characteristics of the liquid crystaldisplay device 20 deteriorate again. Thus, it is preferable to set theretardation of the liquid crystal panel 11 to be larger than about 80nm, more preferably equal to or larger than 82 nm and smaller than about410 nm, more preferably smaller than about 400 nm. It should be notedthat a similar conclusion is obtained also in the case of the liquidcrystal display device of FIGS. 5A and 5B that uses a liquid crystalhaving a positive dielectric anisotropy.

FIGS. 23-28 show the transmittance of the liquid crystal display device20 of FIG. 6A for the front direction while changing the thicknesses dof the liquid crystal layer 12 variously, wherein each of FIGS. 23-28shows the change of the transmittance for each of the three primarycolors, blue (B), green (G) and red (R). In FIGS. 23-28, the change ofthe transmittance is caused by changing the drive voltage from 0 V to6V.

As will be seen clearly from FIGS. 23-26, the transmittance is verysmall for any of the three primary colors even when a drive voltage of 6V is applied, as long as the thickness d of the liquid crystal layer issmaller than about 1 μm (Δn·d=82 nm). See FIG. 23.

When the thickness d of the liquid crystal layer is increased above 1μm, the transmittance increases steeply for all of the three primarycolors. Further, as can be seen clearly in FIGS. 26 and 27, it ispossible to set the transmittance to be generally equal for all of theR, G and B by setting the magnitude of the drive voltage pulse to about4 V.

When the thickness d is increased further as in the case of FIG. 28, inwhich the thickness d is set to 6 μm, the drive voltage that provides agenerally common transmittance for all of the three primary colors isreduced to about 3 V. In this case, however, the range or band of thedrive voltage in which the foregoing common transmittance is obtained issubstantially narrowed as compared with the case of FIG. 26 or 27 inwhich the thickness d is set not to exceed 6 μm. In other words, theresult of FIG. 28 indicates that a small variation of the drive voltagemay cause a coloring of the represented image. In order to avoid such aproblem of unwanted coloring, it is necessary to control the drivevoltage exactly. However, such an exact control of the drive voltage ina mass-produced liquid crystal display device is difficult.

The foregoing analysis indicates that it is preferable to set thethickness d of the liquid crystal layer 12 of FIG. 6A to be larger thanabout 1 μm but not exceeding about 6 μm. Associated with this, it ispreferable to set the retardation of the liquid crystal layer 12 to belarger than about 80 nm but not exceeding about 400 nm. It should benoted that the foregoing conclusion is applicable not only to the liquidcrystal display device of FIGS. 4A and 4B that uses a liquid crystalhaving a negative dielectric anisotropy but also to the liquid crystaldisplay device of FIGS. 5A and 5B that uses a liquid crystal having apositive dielectric anisotropy.

FIGS. 29-33 are CIE-plots (CIE-1931 standard chromaticity diagram)showing the change of the reproduced color observed in the liquidcrystal display device of FIG. 6A for the case in which the polar angleis changed from +80° to −80°. In FIGS. 29-33, the thick continuous lineshows the case in which the azimuth angle is set to 0°, the thincontinuous line shows the case in which the azimuth angle is set to 45°,and the broken line shows the case in which the azimuth angle is set to90°.

Referring to FIG. 29, it should be noted that the observed color changeis minimum for any settings of the polar angle and the azimuth angle aslong as the thickness d of the liquid crystal layer 12 is set to 1 μmand the retardation Δn·d of the liquid crystal panel 11 to 82 nm. Whenthe thickness d of the liquid crystal layer 12 exceeded 3 μm (246 nm interms of the retardation Δn·d of the liquid crystal panel 11) as in thecase of FIG. 30, the observed color change is slightly pronounced.However, azimuth-dependence of the color is still not observed in thecase of FIG. 30.

When the thickness d of the liquid crystal layer 12 has exceeded 4 μm(328 nm in terms of the retardation Δn·d of the liquid crystal panel 11)as in the case of FIG. 31, the observed color change becomes moreprominent. Further, there appears a difference in the color changebetween the case in which the azimuth angle is set to 90° and the casein which the azimuth angle is set to 0° or 450.

When the thickness d of the liquid crystal layer 12 is set to 5 μm (410nm in terms of the retardation Δn·d of the liquid crystal panel 11) asin the case of FIG. 32, or when the thickness d is set to 6 μm (492 nmin terms of the retardation Δn·d) as in the case of FIG. 33, a verylarge color change is observed.

The result of FIGS. 29-33 indicates that it is preferable to set theretardation Δn·d of the liquid crystal layer 12 to be smaller than about300 nm, preferably smaller than 280 nm, which is an intermediate valuebetween the case of FIG. 30 and the case of FIG. 31, when the VA liquidcrystal display device is to be used for a full-color display device ofthe direct-view type, which is required to have wide viewing-anglecharacteristics. It should be noted that the foregoing conclusionapplies not only to the liquid crystal display device of FIGS. 4A and 4Bthat uses a liquid crystal of negative dielectric anisotropy but also tothe liquid crystal display device of FIGS. 5A and 5B that uses a liquidcrystal of positive dielectric anisotropy.

Further, the inventor of the present invention examined the effect ofthe twist angle of the liquid crystal molecules on the viewing-anglecharacteristics of the liquid crystal display device 20 of FIG. 6A. Inthe investigation, the thickness d of the liquid crystal layer 12 is setto 3 μm.

FIGS. 34-36 show the viewing-angle characteristics of the liquid crystaldisplay device respectively for the case in which the twist angle is setto 0°, 90° and 180°. As will be seen from FIGS. 34-36, no substantialdependence of the viewing-angle characteristics on the twist angle isobserved. It should be noted that the foregoing conclusion applies notonly to the liquid crystal display device of FIGS. 4A and 4B that uses aliquid crystal of negative dielectric anisotropy but also to the liquidcrystal display device of FIGS. 5A and 5B that uses a liquid crystal ofpositive dielectric anisotropy.

In the experiments described heretofore about the liquid crystal displaydevice 20 of FIG. 6A, it should be noted that no chiral substance isadded to the liquid crystal layer 12, contrary to the practice used inordinary TN-mode liquid crystal display devices.

FIG. 37 shows the black-mode transmittance of the liquid crystal displaydevice of FIG. 6A for a case in which the polar angle is changed from 0°to 80° in the azimuth direction set to 90°. In the investigation of FIG.37, a liquid crystal of MX941296 (Δn=0.082, Δ∈=−4.6, Merck Japan) isused for the liquid crystal layer 12 in combination with the polarizerof G1220DU (Nitto Denko). The thickness of the liquid crystal layer isset to 3.5 μm and hence the liquid crystal layer 12 has a retardationΔn·d of 287 nm.

As can be seen from FIG. 37, the black-mode transmittance; or thetransmittance of the liquid crystal device in the black representationmode, is minimized by setting the retardation R′ of the retardation film14A in the vicinity of 287 nm. It should be noted that the foregoingconclusion applies not only to the liquid crystal display device ofFIGS. 4A and 4B that uses a liquid crystal of negative dielectricanisotropy but also to the liquid crystal display device of FIGS. 5A and5B that uses a liquid crystal of positive dielectric anisotropy.

Further, the inventor of the present invention has undertaken aninvestigation about the effect of the chiral substance on theviewing-angle characteristics of a VA-mode liquid crystal displaydevice.

In a VA-mode liquid crystal display device such as the device 20 of FIG.6A, the liquid crystal molecules are aligned generally perpendicularlyto the panel substrate as indicated in FIG. 38A in a non-activated statethereof, in which no drive voltage is applied to the liquid crystalpanel. Thus, no substantial effect appears on the viewing-anglecharacteristics even when a chiral substance is added to the liquidcrystal layer 12 forming the liquid crystal panel 11. It should be notedthat FIG. 38A shows the non-activated state of the liquid crystal layer12 with a chiral substance added thereto.

In an activated state shown in FIG. 38B in which the liquid crystalmolecules are aligned horizontally, on the other hand, it is expectedthat the chiral pitch of the chiral substance added to the liquidcrystal layer 12 may induce some effect on the optical property of theliquid crystal display device 20. In the state of FIG. 38B, it should benoted that the liquid crystal molecules show a twisting in the thicknessdirection of the liquid crystal layer 12 with a generally uniform twistangle, which is determined by the chiral pitch p of the chiral substanceand the thickness d of the liquid crystal layer.

In the case in which the chiral substance is not added to the liquidcrystal layer 12, the liquid crystal molecules show a generallyvertically oriented state similar to the state of FIG. 38A in thenon-activated state of the VA-mode liquid crystal display device 20 asindicated in FIG. 39A. However, the liquid crystal molecules show asomewhat irregularly oriented horizontal state in the activated state ofthe liquid crystal display device 20 as indicated in FIG. 39B, due tothe absence of chiral pitch control by the chiral substance. Asindicated in FIG. 39B, the twisting of the liquid crystal moleculesappears in the vicinity of the molecular alignment films carried by thelower and upper substrates 11A and 11B, while no substantial twistingoccurs in a central region C of the liquid crystal layer 12.

FIG. 40 shows the viewing-angle characteristics of the liquid crystaldisplay device 20 of FIG. 6A in which the thickness d of the liquidcrystal layer 12 is set to 3 μm and the twist angle of the liquidcrystal molecules is set to 90°, for the case in which a chiralsubstance is added to the liquid crystal layer 12 with a chiral pitchcontrol in which the d/p ratio is set to 0.25, wherein d represents thethickness of the liquid crystal layer 12 as noted already and prepresents the chiral pitch of the chiral substance.

Referring to FIG. 40, it should be noted that the region that provides acontrast ratio CR of 10 or more is decreased as compared with theviewing-angle characteristics of FIG. 35 for a comparable constructionof the liquid crystal display device 20 except that no chiral substanceis added to the liquid crystal layer 12. The result of FIG. 40 indicatesthat the use of chiral substance in a VA-mode liquid crystal displaydevice is not preferable from a viewpoint of improving the viewing-anglecharacteristics.

FIGS. 41 and 42 show the transmittance of the liquid crystal displaydevice 20 for each of the three primary colors R, G and B in the frontdirection of the display device for a case in which the thickness d isset to 3 μm and the twist angle of the liquid crystal molecules is setto 90°, wherein FIG. 41 shows the case in which a chiral substance isadded while FIG. 42 shows the case in which no chiral substance isadded.

The result of FIGS. 41 and 42 indicates that the addition of the chiralsubstance causes a decrease of the transmittance and hence thebrightness of the liquid crystal display device 20. It is believed thatthe region C of FIG. 39B, in which the liquid crystal molecules are nottwisted, causes an efficient rotation of the optical plane for theoptical beam passing therethrough, while no such a region is formed inthe state of FIG. 38B.

From FIGS. 41 and 42, it is concluded that it is preferable not to add achiral substance to the liquid crystal layer in a VA-mode liquid crystaldisplay device from a viewpoint of improving the brightness and hencethe contrast ratio. It should be noted that the foregoing conclusionapplies not only to the liquid crystal display device of FIGS. 4A and 4Bthat uses a liquid crystal of negative dielectric anisotropy but also tothe liquid crystal display device of FIGS. 5A and 5B that uses a liquidcrystal of positive dielectric anisotropy.

Further, the inventor of the present invention has conducted aninvestigation on the effect of the pretilt angle of the liquid crystalmolecules on the viewing-angle characteristics of the liquid crystaldisplay device 20 of FIG. 6A. The result is represented in FIGS. 43-47,wherein FIG. 43 shows the case in which the pretilt angle is set to89.99°, FIG. 44 shows the case in which the pretilt angle is set to 85°,FIG. 45 shows the case in which the pretilt angle is set to 80°, andFIG. 46 shows the case in which the pretilt angle is set to 75°.Further, FIG. 47 shows the viewing-angle characteristics of a standardTN-mode liquid crystal display device as a reference.

Referring to FIGS. 43-47, it should be noted that the case of FIG. 43,in which the pretilt angle is set substantially to 90°, provides thewidest viewing-angle characteristics and that the viewing-anglecharacteristics become narrower with decreasing pretilt angle. When thepretilt angle is set to 75° as in the case of FIG. 46, the obtainedviewing-angle characteristics are more or less equal to that of atypical TN-mode liquid crystal display device shown in FIG. 47.

The foregoing results indicate that it is preferable to set the pretiltangle of the liquid crystal molecules to be larger than 75°, preferablylarger than 87°, more preferably larger than 89°. It should be notedthat the foregoing conclusion applies not only to the liquid crystaldisplay device of FIGS. 4A and 4B that uses a liquid crystal of negativedielectric anisotropy but also to the liquid crystal display device ofFIGS. 5A and 5B that uses a liquid crystal of positive dielectricanisotropy.

First Embodiment

FIG. 48 shows a construction of a liquid crystal display device 30according to a first embodiment of the present invention in across-sectional view.

Referring to FIG. 48, the liquid crystal display device 30 includes aglass substrate 31A and a glass substrate 31B, wherein the glasssubstrate 31A carries, on an upper major surface thereof, a transparentelectrode 31 a′ of ITO and a molecular alignment film 31 a covering theelectrode 31 a′ as usual in a liquid crystal display device. Similarly,the glass substrate 31B carries, on a lower major surface thereof, atransparent electrode 31 b′ of ITO and a molecular alignment film 31 bcovering the electrode 31 b′, wherein the substrate 31A and thesubstrate 31B are disposed such that the molecular alignment film 31 aand the molecular alignment film 31 b face with each other with polymerspacer balls 31 c intervening therebetween.

Further, the space thus formed between the substrates 31A and 31B issealed by providing a seal member (not illustrated), and a liquidcrystal having a negative dielectric anisotropy such as MJ941296 of E.Merck, Inc. (Δn=0.0804, Δ∈=−4) is injected to the foregoing space by avacuum injection process. Thereby, a liquid crystal layer 32 is formed.In such a liquid crystal panel, the thickness d of the liquid crystallayer 32 d is determined by the diameter of the polymer spacer balls 31c.

On the outer sides of the liquid crystal panel thus formed, retardationfilms 33A and 33B are disposed. Further, polarizers 34A and 34B aredisposed on the outer sides of the retardation films 33A and 33B withrespective orientations with respect to the center of twist, asexplained already with reference to FIG. 1 or FIG. 6A. In other words,the liquid crystal display device 30 of FIG. 48 corresponds to the caseof the liquid crystal display device 20 of FIG. 6A in which theretardation film 14B is provided.

TABLE I below summarizes the result of an evaluation test conducted onthe liquid crystal display device 30 for the response and viewing-anglecharacteristics at 25° C. for various thicknesses d of the liquidcrystal layer 32 while setting the twist angle to 45°. In thisexperiment, RN783 of Nissan Chemicals KK was used for the molecularalignment t films 31 a and 31 b. Further, G1220DU of Nitto Denko KK orSK-1832A of Sumitomo Chemicals KK was used for the polarizers 34A and34B. In the tested device 30, the retardation films 33A and 33B wereomitted. However, the compensation of the retardation of the liquidcrystal panel was achieved, to some extent, by protective films coveringthe polarizers. The protective film is known as TAC film (TAC=triacetatecellulose) and has a very small, but finite birefringence. For example,the G1220DU polarizer carries a protective film that shows a retardationof about 44 nm. The TAC film of the SK-1832AP7 polarizer exhibits aretardation of about 50 nm. No chiral substance was added to the liquidcrystal layer 32.

TABLE I T_(on) T_(off) CR ≧ 10° at 25° C. PANEL # d (μm) (ms) (ms) 0°90° 180° −90° av. OM480 3.75 13.56 9.04 41 54 48 54 49.25 OM482 3.008.79 5.71 42 58 52 58 52.50 OM484 2.60 7.81 4.45 42 60 52 60 53.50

Referring to TABLE I, it should be noted that the turn-on transient timeT_(on) as well as the turn-off transient time T_(off) of the liquidcrystal display device 30 decreases with decreasing thickness d of theliquid crystal layer 32. In other words, the response of the liquidcrystal display device 30 improved by decreasing the thickness d of theliquid crystal layer 32. Further, the range of the viewing-angle inwhich the contrast ratio R exceeds 10° (CR≧10°) expands with decreasingthickness d of the liquid crystal layer 32. On the other hand, excessivedecrease of the thickness d results in a decrease of the brightness asalready noted. Thus, it is preferred to set the thickness d of theliquid crystal layer 32 such that the retardation Δn·d of the liquidcrystal layer 32 falls in a range between about 80 nm and about 400 nm.

It should be noted that the foregoing TAC film is used extensively as aprotective film of polarizer or analyzer in conventional TN or STNliquid crystal display devices due to the very small retardation value.A typical TAC film has a positive retardation R′ of 5-15 nm in thein-plane direction and a negative retardation of 38-50 nm in thethickness direction. The value of the retardation R or R′ can be changedby changing the thickness of the film.

On the other hand, the inventor of the present invention has discoveredthat the VA-mode liquid crystal display device of the present inventionis susceptible to such a very small retardation with regard to theviewing-angle and contrast and that an optimization is necessary alsofor the TAC film. Further, it was discovered that such an optimizationof the TAC film can lead to a further improvement of the viewing-anglecharacteristics of the liquid crystal display device. The TAC film onthe outer surface of the polarizer does not affect the opticalproperties of the liquid crystal display device.

In conventional TN or STN liquid crystal display devices, the TAC filmis provided with an orientation such that the retardation thereof axisextends in a direction parallel to the absorption axis of the polarizeror analyzer adjacent to the TAC film. On the other hand, the inventor ofthe present invention has discovered, as will be described later indetail, that it is preferable to dispose the TAC film such that theretardation axis thereof intersects generally perpendicularly to theabsorption axis of the adjacent polarizer or analyzer. By doing so, theeffective retardation of the retardation film is given as a differencebetween the positive retardation of the retardation film and thepositive retardation of the TAC film.

Thus, in the case in which a standard polarizer carrying a TAC filmthereon is to be used, it is necessary to set the retardation of theretardation film larger than the desired retardation by an amountcorresponding to the retardation of the TAC films disposed on both sidesof the liquid crystal panel. On the other hand, when a polarizer thatcarries a TAC film thereon with such an orientation that the retardationaxis of the TAC film extends parallel to the absorption axis of thepolarizer, the effective retardation of the polarizer increases, and itis necessary to set the retardation of the retardation film to besmaller than the desired retardation by an amount corresponding to theretardation of the TAC films disposed on both sides of the liquidcrystal panel.

FIGS. 49A and 49B show the viewing-angle characteristics of the liquidcrystal display device 30 of FIG. 48 for the case in which the thicknessd is set to 3 μm and the twist angle is set to 45°. In the example ofFIGS. 49A and 49B, no chiral substance was added to the liquid crystallayer 32. Further, the TAC films covering the polarizers 34A and 34Bwere used for the retardation films 33B and 34B. In other words, noseparate retardation films were used. In the experiment, the G1220DUpolarizer marketed by Nitto Denko KK. was used for the polarizers 34Aand 34B as already noted, in combination with the MJ941296 liquidcrystal of Merck Japan, LTD.

In FIG. 49A, it should be noted that a region indicated by whiterepresents the viewing-angle characteristics that provide a contrastratio equal to 10 or more (CR≧10). It will be noted that a very largearea is represented white in FIG. 49A, indicating that the tested liquidcrystal display 30 device provides an excellent viewing-anglecharacteristics. Further, FIG. 49B indicates that a contrast ratio ofnear 2000 is obtained in the front direction of the liquid crystaldisplay device.

FIGS. 50A and 50B show the viewing-angle characteristics of the liquidcrystal display device 30 of FIG. 48 for the case in which acommercially available retardation film (VAC0 of Sumitomo Chemicals KK)is used for the retardation films 33A and 33B, wherein it should benoted that the retardation films 33A and 33B are set such that a totalretardation R′ including also the contribution from the TAC films of thepolarizers 34A and 34B, takes a value of 218 nm, which value is selectedclose to the retardation Δn·d of 241 nm of the liquid crystal layer 12and hence the liquid crystal panel 11.

As will be seen from FIG. 50A, the area of the viewing-angle thatprovides a contrast ratio of 10 or more increases further as comparedwith the case of FIG. 49A. Further, the contrast achieved in the frontdirection of the panel reaches 4000 as indicated in FIG. 50B.

It has been described previously with reference to FIGS. 43-47 that theviewing-angle characteristics of a VA-mode liquid crystal display deviceare deteriorated to the degree of an ordinary TN-mode liquid crystaldisplay device when the pretilt angle is set to 75°. In the constructionof FIG. 48 that includes the retardation films 33A and 33B above andbelow the liquid crystal layer 32, however, the area of theviewing-angle in which the contrast ratio CR of 10 or more is achievedis increased to a satisfactory level for a liquid crystal display deviceas indicated in FIG. 51. It should be noted that the result of FIG. 51is for the case in which the liquid crystal layer 32 has a thickness of3 μm and the pretilt angle is set to 75°.

Second Embodiment

Next, a second embodiment of the present invention will be described.

In the second embodiment, another liquid crystal, MX95785 of MerckJapan, Ltd., is used in the liquid crystal display device 30 of FIG. 48for the liquid crystal layer 32, in place of the foregoing MJ941296liquid crystal. The MX95785 liquid crystal has a birefringence Δn of0.813 and a negative dielectric anisotropy Δ∈ of −4.6. As the rest ofthe construction is identical to the liquid crystal display device 30 ofFIG. 45, further description about the construction of the liquidcrystal display device will be omitted.

FIG. 52 shows turn-on transient characteristics of the liquid crystaldisplay device for the case in which the thickness d of the liquidcrystal layer 32 is set to 3 μm, wherein FIG. 52 shows a turn-ontransient time T_(on) for each of the twist angles of 0°, 45° and 90°.No chiral substance is added to the liquid crystal layer 32. As will beseen clearly from FIG. 49, the turn-on transient time T_(on) is about 10ms except for the case where the twist angle is 0°, as long as the drivevoltage is in the range of 4-8 V. In other words, the liquid crystaldisplay device 30 shows an excellent turn-on response as compared withconventional TN-mode liquid crystal display devices that typically showa turn-on time T_(on) of 20 ms or more.

FIG. 53 shows the turn-off transient characteristics of the liquidcrystal display device 30 for the case where the thickness d of theliquid crystal layer 32 is set to 3 μm, wherein FIG. 53 shows a turn-oftransient time T_(off) for each of the twist angles of 0°, 45° and 90°.In this example, as well, no chiral substance is added to the liquidcrystal layer 32. As will be seen clearly from FIG. 53, the turn-offtransient time T_(off) is about 5 ms irrespective of the twist angle ofthe liquid crystal molecules. In other words, the liquid crystal displaydevice 30 shows an excellent turn-off response as compared withconventional TN-mode liquid crystal display devices that typically showa turn-off time T_(off) of 40 ms or more.

TABLE II R′ (nm) VAC + CR ≧ 10 at 25° C. inversion of gradation TAC 0°90° 180° −90° av. 0° 45° 90° 135° 180° av.  88 43 60 52 61 54 40 40 5060 38 46 185 42 70 57 66 59 30 40 70 66 36 49 282 38 58 52 58 52 26 4470 70 38 50 _(Δ)n · d = 246 nm

TABLE II shows, in the left column, the viewing-angle of the liquidcrystal display device 30 of the present embodiment for various valuesof the negative retardation R′ caused by the polarizers 34A and 34B aswell as by the retardation films 33A and 33B. Further, TABLE II shows,in the right column, the viewing-angle in which an inversion occurs in ahalf-tone image displayed with an eleven-step gradation in the frontdirection of the liquid crystal panel. With increasing polar angle fromthe front direction, there occurs an inversion in the gradation, whilesuch an inversion of gradation deteriorates the quality of the displayedimage seriously. In the experiment of TABLE II, it should be noted thatthe liquid crystal layer 32 has a positive retardation with a magnitudeof 246 nm. From TABLE II, it should be noted that the area of thesatisfactory viewing-angle increases for all of the azimuth angles of90°, −90° and 180°, by setting the retardation caused by the retardationfilms 33A and 33B as well as by the TAC films of the polarizers 34A and34B to be generally equal to the retardation Δn·d of the liquid crystallayer 32.

TABLE III twist CR ≧ 10 at 25° C. inversion of gradation (°) 0° 90° 180°−90° av. 0° 45° 90° 135° 180° av.  0 44 60 49 60 53 40 40 52 60 38 46 4543 60 52 61 54 40 40 50 60 38 46 90 41 59 50 60 53 40 40 54 64 32 46 noVAC films, use G1220DU polarizer (R′ = 88 nm)

TABLE III shows, in the left column, the viewing-angle of the liquidcrystal display device of the present embodiment for various twistangles of the liquid crystal molecules in the liquid crystal layer 32.Further, the right column of TABLE III represents the viewing-angle inwhich an inversion occurs in a half-tone image displayed with aneleven-step gradation in the front direction of the liquid crystalpanel, similarly to the case of TABLE II. TABLE III indicates that thereis no substantial effect caused in the viewing-angle characteristics ofthe liquid crystal display device 30 by the twist angle of the liquidcrystal molecules. It should be noted that the result of TABLE III isfor the case in which the retardation films 33A and 33B are omitted andthe phase compensation of the optical beam is achieved only by theretardation R′ of 88 nm, which is caused by the TAC films covering thepolarizers 34A and 34B.

Third Embodiment

FIG. 54 shows a construction of a liquid crystal display device 40according to a third embodiment of the present invention, wherein thoseparts described previously are designated by the same reference numeralsand the description thereof will be omitted.

Referring to FIG. 54, the liquid crystal display device 40 has aconstruction similar to that of the liquid crystal display device 30 ofFIG. 48, except that the retardation film 33B of FIG. 48 having anegative retardation is replaced by a first retardation film (33B)₁having a positive retardation and a second retardation film (33B)₂having a negative retardation, wherein the first retardation film (33B)₁of the positive retardation is disposed closer to the liquid crystalpanel 31 with respect to the second retardation film (33B)₂ of thenegative retardation. Thereby, it should be noted that the secondretardation film (33B)₂ has an optical axis extending perpendicularly tothe principal surface of the liquid crystal panel 31, while the firstretardation film (33B)₁ has an optical axis parallel to the principalsurface of the liquid crystal panel 31.

FIG. 55 shows a black-mode transmittance representing the transmittanceof the liquid crystal display device 40 of FIG. 54 for the non-activatedstate thereof in which no drive voltage is applied to the liquid crystalcell, wherein FIG. 55 shows the black-mode transmittance as viewed inthe direction where the azimuth angle is 90′ for the case in which thethickness d of the liquid crystal layer 32 is set to 3.5 μm and thetwist angle is set to 45′. In the example of FIG. 55, the negativeretardation of the retardation film (33B)₂ is set generally equal to theretardation Δn·d of the liquid crystal panel 31 and the positiveretardation of the retardation film (33B)₁ is fixed at 100 nm. Thereby,FIG. 55 shows the change of the black-mode transmittance as a functionof the optical-axis angle θ, wherein the optical-axis angle θ indicatesthe angle that the optical axis of the retardation film (33B)₁ formswith respect to the center of twist of the liquid crystal molecules asindicated in FIG. 54.

Referring to FIG. 55, it should be noted that the black-modetransmittance becomes minimum for all of the polar angles when theoptical-axis angle θ is set to about 45°. In other words, it is possibleto improve the viewing-angle characteristics for all of the polar anglesby setting the optical-axis angle θ to be about 45°. Further, thecontrast ratio CR is maximized as a result of minimization of theblack-mode transmittance.

In FIG. 55, it should be noted further that a minimum of the black-modetransmittance is achieved also for the polar angle of 0° or 20° when theoptical-axis angle θ is set to about 135°. However, this state is not atrue optimum, as the black-mode transmittance is not minimized for thepolar angles of 40° or more in this state.

FIG. 56 is a diagram showing the black-mode transmittance of the liquidcrystal display device 40 of FIG. 54 for various polar angles as afunction of the positive retardation R of the retardation film (33B)₁.In FIG. 56, as well, the azimuth angle is set to 90°.

Referring to FIG. 56, it should be noted that the black-modetransmittance is minimized for all of the polar angles by setting theretardation R of the retardation film (33B)₁ to fall in a range between20 nm and 60 nm. By optimizing the retardation R of the retardation film(33B)₁ as such, the black-mode transmittance can be reduced to 0.002 orless.

FIG. 57 shows the viewing-angle characteristics of the liquid crystaldisplay device 40 of FIG. 54 for a case in which the retardation R ofthe positive retardation film (33B)₁ is set to 25 nm and the retardationR′ of the negative retardation film (33B)₂ is set to 240 nm. Further,the twist angle of the liquid crystal molecules is set to 45° and thethickness of the liquid crystal layer 32 is set to 3 μ.

As will be understood from FIG. 57, a very wide viewing-angle isobtained for the liquid crystal display device 40 by combining thepositive and negative retardation films.

When the same positive and negative retardation films are disposed witha reversed order, on the other hand, it was discovered that theviewing-angle characteristics of the liquid crystal display device 40 isdeteriorated substantially as indicated in FIG. 58. The result of FIG.58 indicates that the order of the positive and negative retardationfilms is essential for improving the view angle characteristics of theliquid crystal display device.

FIG. 59 shows the viewing-angle characteristics of the liquid crystaldisplay device 40 of FIG. 54 for the case in which the retardation filmsare omitted. As can be seen clearly in FIG. 59, the viewing-angle innarrowed substantially when the retardation films are eliminated.

Fourth Embodiment

FIG. 60 shows a construction of a liquid crystal display device 50 ofthe fourth embodiment, wherein those parts described previously aredesignated by the corresponding reference numerals and the descriptionthereof will be omitted.

Referring to FIG. 60, it should be noted that the liquid crystal displaydevice 50 has a construction similar to that of the liquid crystaldisplay device 40 of FIG. 54, except that a retardation film (33A)₂having a negative retardation is provided further in the gap formedbetween the lower polarizer 34A and the liquid crystal panel 31.

FIG. 61 shows the black-mode transmittance of the liquid crystal displaydevice 40 as a function of the retardation of the retardation film(33B)₁ for a case in which the total retardation of the foregoingnegative retardation film and the retardation film (33B)₁ is set to begenerally equal to the retardation of the liquid crystal panel 31.

As will be understood from FIG. 61, the black-mode transmittance becomesminimum when the retardation of the retardation film (33B)₁ is in therange of 50-60 nm. Thus, in order that the retardation film (33B)₁ ismost effective for increasing the contrast ratio, it is necessary to setthe retardation of the retardation film (33B)₁ to be below about 100 nm.

FIG. 62 shows the black-mode transmittance of the liquid crystal displaydevice 50 of FIG. 60 for a case in which the retardation of theretardation film (33B)₁ is set to 30 nm and the retardation R′ of thenegative retardation films (33B)₂ and (33A)₂ is changed variously.Similarly as before, the evaluation is made in the direction in whichthe azimuth angle is 90°, while changing the polar angle variously.

As will be understood from FIG. 62, the minimum of the black-modetransmittance is obtained when the negative retardation R′ formed by theretardation film (33B)₂ is about 250 nm, while this optimum value isslightly smaller than the retardation Δn·d of the liquid crystal layer32. As explained previously, the optimum retardation of the retardationfilm (33B)₁ is equal to the retardation Δn·d of the liquid crystal layer32 when the positive retardation film (33B)₁ is not provided. Thus, whenthe positive retardation film (33B)₁ is used in addition to the negativeretardation films (33B)₂ and (33A)₂, the optimum value of retardation ofthe negative retardation film (33B)₂ should be set slightly smaller thanthe retardation Δn·d of the liquid crystal layer 32. In any case, it isnecessary to set the total retardation R′ of the negative retardationfilm to be smaller than twice the retardation Δn·d of the liquid crystallayer 32 when the retardation film (32B)₂ alone is used or when anothernegative retardation film is used.

FIG. 63 shows the viewing-angle characteristics of the liquid crystaldisplay device 50 of FIG. 60.

In FIG. 63, it should be noted that the area in which the contrast ratioexceeds 10 is increased as compared with the result of FIG. 19 in whichonly the negative retardation film is used.

Fifth Embodiment

FIG. 64 shows the construction of a liquid crystal display device 50′according to a fifth embodiment of the present invention, wherein thoseparts corresponding to the parts described previously are designated bythe same reference numerals and the description thereof will be omitted.

Referring to FIG. 64, it will be noted that the liquid crystal displaydevice 50′ includes a positive retardation film (33A)₁ between theliquid crystal panel 31 and the negative retardation film (33A)₁ andprovides an excellent viewing-angle characteristics as represented inFIG. 65.

Sixth Embodiment

FIG. 66 shows a construction a liquid crystal display device 60according to a fifth embodiment of the present invention, wherein thoseparts described previously are designated by the same reference numeralsand the description thereof will be omitted.

Referring to FIG. 66, the liquid crystal display device 60 has aconstruction similar to that of the liquid crystal display device 500R50′ explained previously, except that the positive retardation film(33B)₁ and the negative retardation film (33B)₂ are replaced by a singlebiaxial retardation film 33B′ in the liquid crystal display device 60 ofthe present embodiment.

The biaxial retardation film 33B′ has refractive indices n_(x), n_(y)and n_(z) respectively in the x-, y- and z-directions, wherein thereholds a relationship n_(x)>n_(y)>n_(z) or n_(y)>n_(x)>n_(z). Such abiaxial retardation film itself is known for example from the JapaneseLaid-open Patent Publication 59-189325.

It should be noted that the biaxial retardation film 33B′ forms aretardation in the plane of the film 33B′ with a magnitude representedby |n_(x)−n_(y)|·d and further a retardation in the normal direction orthickness direction of the film 33B′ with a magnitude represented by{(n_(x)+n_(y))/2+n_(z)}·d. In the present embodiment, an optimum resultis obtained by setting the foregoing in-plane retardation to be 120 nmor less and the retardation in the thickness direction to be generallyequal to the retardation Δn·d of the liquid crystal layer 32. In theexample of FIG. 66, it should be noted that the retardation film 33B′ isdisposed such that an in-plane retardation axis is generally parallel tothe absorption axis of the adjacent polarizer 34B, wherein the in-planeretardation axis represents the direction in which the retardationbecomes maximum. In the case where the relationship n_(x)>n_(y)>n_(z)holds, the in-plane retardation axis coincides with the x-axis, while inthe case where the relationship n_(y)>n_(x)>n_(z) holds, the in-planeretardation axis coincides with the y-axis.

FIG. 67 shows the black-mode transmittance of the liquid crystal displaydevice 60 of FIG. 66 for a case in which the azimuth angle of thein-plane retardation axis n_(x) of the biaxial retardation film 33B′ ischanged variously.

As will be noted from FIG. 67, the black-mode transmittance can beminimized by disposing the biaxial retardation film 33B′ with such anorientation that the azimuth angle θ of the in-plane retardation axisn_(x) is about 45° or about 135°, in other words, the retardation axisn_(x) extends perpendicularly or parallel to the absorption axis of theadjacent polarizer 34B. Particularly, it should be noted that theblack-mode transmittance can be suppressed below about 0.2% or less forthe polar angles between 80°-0°, by setting the foregoing azimuth angleθ to be about 45°.

FIG. 68 shows the black-mode transmittance of the liquid crystal displaydevice 60 of FIG. 66 for the case in which the thickness of the biaxialretardation film 33B′ is changed.

As can be seen from FIG. 68, the black-mode transmittance becomesminimum when the thickness of the liquid crystal layer is set to about130 μm, while it should be noted that the biaxial retardation film 33B′having the foregoing thickness of 130 μm forms a retardation R or R′ ofabout 39 nm within the plane of the film 33B′ and about 240 nm in thedirection perpendicular to the film 33B′.

Generalizing the foregoing, it is concluded that the in-planeretardation R of the liquid crystal display device 60 of FIG. 66 ispreferably set to be smaller than about 120 nm, more preferably in therange of 20-60 nm, and that the retardation R′ in the thicknessdirection is set equal to or smaller than about twice the retardationΔn·d of the liquid crystal layer 32.

FIG. 69 shows the view angle characteristics of the liquid crystaldisplay device 60 of FIG. 66, wherein it is set in FIG. 69 thatn_(x)=1.502, n_(y)=1.5017, n_(z)=1.5 and d=120 nm, wherein d representsthe thickness of the liquid crystal layer 32. As can be seen from FIG.69, the liquid crystal display device 60 exhibits an excellent viewangle characteristic.

A biaxially tensioned polycarbonate film such as the VAC film suppliedfrom Sumitomo Chemicals or a TAC film used for the protective film ofpolarizers may be used for the biaxial retardation film.

Seventh Embodiment

FIG. 70 shows the construction of a liquid crystal display device 70according to a seventh embodiment of the present invention, whereinthose parts corresponding to the parts described previously aredesignated by the same reference numerals and the description thereofwill be omitted.

Referring to FIG. 70, the present embodiment uses, in addition to theretardation film 33B′, another optically biaxial retardation film 33A′between the liquid crystal panel 31 and the polarizer 34A, wherein theretardation films 33B′ and 33A′ are disposed such that the retardationaxis of the film 33B′ intersects substantially perpendicularly to theabsorption axis of the adjacent analyzer 34B and such that theretardation axis of the film 33A′ intersects substantiallyperpendicularly to the absorption axis of the adjacent polarizer 34A.

FIG. 71 shows the view angle characteristics of the liquid crystaldisplay device 70. As can be seen from FIG. 71, the liquid crystaldisplay device 70 shows an excellent viewing-angle characteristic.

Eighth Embodiment

FIG. 72 shows the construction of a liquid crystal display device 80according to an eighth embodiment of the present invention, whereinthose parts corresponding to the parts described previously aredesignated by the same reference numerals and the description thereofwill be omitted.

Referring to FIG. 72, it will be noted that the liquid crystal displaydevice 80 of the present embodiment has a construction similar to thatof the liquid crystal display device 40 of FIG. 54 except that theretardation film (33B)₂ is omitted.

FIG. 73 shows the black-mode transmittance of the liquid crystal displaydevice 80 for various azimuth angles of the positive retardation film(33B)₁ and hence the direction of the retardation axis n_(x).

As can be seen in FIG. 73, the black-mode transmittance of the liquidcrystal display device of the liquid crystal device 80 becomes minimumwhen the axis n_(x) intersects the twist central axis with an angle ofabout 45° or about 135°. Particularly, the angle of 45° is preferable inview point of minimization of the transmittance for the polar angles of0-80°.

FIG. 74 shows the black-mode transmittance of the liquid crystal displaydevice 80 as a function of the thickness of the positive retardationfilm (33B)₁.

Referring to FIG. 74, the black-mode transmittance of the liquid crystaldisplay device becomes minimum when the retardation film (33B)₁ has athickness of 140-150 nm. Further, the in-plane retardation R of theretardation film (33B)₁ falls in the range of 140-160 μm when thethickness of the film (33)₁ is in the range of 140-160 μm. Thus, whenthe positive retardation film (33B)₁ alone is to be used in the liquidcrystal display device 80, the in-plane retardation of the film (33B)₁is preferably set to 300 nm or less.

FIG. 75 shows the viewing-angle characteristics of the liquid crystaldisplay device 80 optimized according to the teaching of FIGS. 73 and74.

As can be seen from FIG. 75, the viewing-angle characteristic of theliquid crystal display device 80 is improved substantially as comparedwith the case of FIG. 59 in which no retardation film is provided.

Ninth Embodiment

FIG. 76 shows the construction of a liquid crystal display device 90according to a ninth embodiment of the present invention, wherein thoseparts corresponding to the parts described previously are designated bythe same reference numerals and the description thereof will be omitted.

Referring to FIG. 76, the liquid crystal display device 90 has aconstruction similar to the liquid crystal display 80 of FIG. 72 exceptthat the positive retardation film (33A)₁, used in the liquid crystaldisplay device 50′ of FIG. 64, is added. In the construction of FIG. 76,it should be noted that the retardation film (33B)₁ is disposed suchthat the in-plane retardation axis n_(x) intersects perpendicularly tothe absorption axis of the analyzer 34B that is located adjacent to theretardation film (33B)₁ and such that the in-plane retardation axisn_(x) of the retardation film (33A)₁ intersects the absorption axis ofthe adjacent polarizer 34B perpendicularly.

FIG. 77 shows the viewing-angle characteristics of the liquid crystaldisplay device 90.

Referring to FIG. 77, the viewing-angle characteristic of the liquidcrystal display device 90 is improved substantially as compared with thecharacteristic of FIG. 59 for the case in which no retardation film isprovided.

Tenth Embodiment

FIG. 78 shows the construction of a liquid crystal display device 100according to a tenth embodiment of the present invention, wherein thoseparts corresponding to the parts described previously are designated bythe same reference numerals and the description thereof will be omitted.

Referring to FIG. 78, the liquid crystal display device 100 has aconstruction similar to that of the liquid crystal display device 90explained previously, except that the retardation film (33B)₁ isdisposed such that the in-plane retardation axis n_(x) intersects theabsorption axis of the adjacent analyzer 34B with an angle of 45° andthat the retardation film (33A)₁ is disposed such that the in-planeretardation axis n_(x) of the retardation film (33A)₁ intersects theabsorption axis of the adjacent polarizer 34A with an angle of 45°.

FIG. 79 shows the viewing-angle characteristics of the liquid crystaldisplay device 100 for a case in which the retardation films (33A)₁ and(33B)₁ provide a retardation R of 75 nm.

As will be understood from FIG. 79, the viewing-angle characteristic ofthe liquid crystal display device 100 is slightly inferior to the otherembodiments, although the viewing-angle characteristic of FIG. 79 isimproved over the viewing-angle characteristic of FIG. 59 in which theretardation film is not provided.

Eleventh Embodiment

FIG. 80 shows a construction of a liquid crystal display device 110 ofan active-matrix type.

Referring to FIG. 80, the liquid crystal display device 110 has aconstruction similar to that of FIG. 48, except that a plurality oftransparent pixel electrodes (31 a′)_(PIXEL) and corresponding thin-filmtransistors (31 a′)_(TFT) that drive the pixel electrodes, are providedon the glass substrate 31A or 31B, in correspondence to pixels that aredefined in the liquid crystal panel 31. Thus, the transparent pixelelectrode (31 a′)_(PIXEL) and the thin-film transistor (31 a′)_(TFT)correspond to the electrode 31 a′ or electrode 31 b′ of FIG. 48.Further, a data bus DATA and an address bus ADDR extend on the substrate31A or 31B respectively for supplying a drive signal to the thin-filmtransistors forming the matrix array and for selectively activating thethin-film transistors in the array.

FIG. 81 shows the viewing-angle characteristics of the liquid crystaldisplay device 110 of FIG. 81 for the case in which the MJ95785 liquidcrystal of Merck Japan, LTD. is used for the liquid crystal layer and inwhich the liquid crystal layer is formed to have a thickness of 3 μm. InFIG. 81, it should further be noted that the twist angle of the liquidcrystal molecules is set to 45° and the liquid crystal layer shows aretardation Δn·d of 241 nm. Further, the RN 783 film of NissanChemicals, KK. is used for the molecular alignment films 31 a and 31 b.As will be understood clearly from FIG. 81, the active-matrix liquidcrystal display device exhibits a very wide viewing-anglecharacteristic.

Twelfth Embodiment

In the embodiments described heretofore, each of the pixels in theliquid crystal display device has a so-called single-domain structureshown in FIGS. 82A-82C, in which the molecular alignment of the liquidcrystal molecules is uniform in each of the pixels. In FIGS. 82A-82C,those parts corresponding to the parts described previously aredesignated by the same reference numerals and the description thereofwill be omitted.

Referring to FIGS. 82A-82C, it should be noted that FIG. 82A shows onepixel in the liquid crystal display device in a plan view, while FIG.82B shows the cross sectional view of the pixel taken along a line A-Bin FIG. 82A in an activated state of the liquid crystal display device.Further, FIG. 82C shows the state in which the liquid crystal displaydevice is irradiated by optical beams X and Y from two directions. Itshould be noted that FIG. 82A shows the rubbing direction of themolecular alignment film 31 b provided on the upper substrate 31B by acontinuous line. Further, the rubbing direction of the molecularalignment film 31 a on the lower substrate 31A is represented in FIG.82A by a dotted line. The continuous line and the dotted line intersecteach other with an angle α₁, wherein the angle α₁ is set to 45° when thetwist angle of the liquid crystal molecules is to be set to 45°.

As can be seen in FIG. 82C, the molecular alignment of the liquidcrystal molecules as viewed in the traveling direction of the opticalbeam changes, in the activated state of the liquid crystal displaydevice, depending on whether the optical beam travels along the path Xor along the path Y. When there exists such an asymmetricity in theoptical structure of the liquid crystal display device, the problem ofdeterioration of the viewing-angle characteristics is inevitable.

FIGS. 83A-83C show a construction of a liquid crystal display device 120according to a seventh embodiment of the present invention, whereinthose parts described previously are designated by the same referencenumerals and the description thereof will be omitted. It should be notedthat FIG. 83A shows a plan view similar to the plan view of FIG. 82A,while FIGS. 83B and 83C show cross-sectional views corresponding toFIGS. 82B and 82C.

Referring to FIGS. 83A-83C, it should be noted that the presentembodiment uses ultraviolet-reformed molecular alignment films 31 a′ and31 b′ such that the molecular alignment films 31 a′ and 31 b′ cover apart of the molecular alignment film 31 a and a part of the molecularalignment film 31 b, respectively. Such ultraviolet-reformed molecularalignment films may be formed by depositing a molecular alignment filmforming the films 31 a′ and 31 b′ on the molecular alignment film 31 aor 31 b, after a rubbing process of the film 31 a or 31 b is completed.Further, the molecular alignment film thus deposited is exposed to anultraviolet radiation such that the molecules in the molecular alignmentfilm thus deposited cause a desired alignment. After such an alignmentof the molecules, the deposited molecular-alignment film is patternedsuch that only a part thereof remains on the underlying molecularalignment film 31 a or 31 b.

By forming the molecular alignment film 31 a′ in the lower part of thepixel and by forming the molecular alignment film 31 b′ in the upperpart of the pixel in the illustration of FIG. 83C, the optical beamtraveling in the direction X and the optical beam traveling in thedirection Y experience substantially the same effect of molecularorientation of the liquid crystal molecules. In other words, the liquidcrystal display device shows an optical property that is substantiallyidentical in the X-direction and in the Y-direction.

FIGS. 84A-84C show a modification of the present embodiment.

Referring to FIG. 84A, the direction of rubbing is changed in the pixelin the upper part and lower part in the illustration of FIG. 84A, andthus, the molecular orientation is different in the right region andleft region of the pixel as can be seen in the cross-sectional view ofFIG. 84B. As noted in FIG. 84A, the rubbing directions of the upper andlower molecular alignment layers 31 a and 31 b cross with each otherwith an angle α₁ in the upper part of the pixel while the rubbingdirections cross with each other with an angle α₂ in the lower part ofthe pixel. As a result, the optical beam traveling in the X-directionand the optical beam traveling in the Y-direction experiencesubstantially the same effect of molecular orientation of the liquidcrystal molecules. Thereby, the viewing-angle characteristics of theliquid crystal display device are improved substantially.

FIG. 85 shows the viewing-angle characteristics of the liquid crystaldisplay device of FIG. 84 for the case in which the angles α₁ and α₂ areboth set to 45°, in which the MJ95785 liquid crystal is used for theliquid crystal layer 32. The thickness d of the liquid crystal layer 32is set to 3 μm. No chiral substance is added to the liquid crystal layer32. Thus, the liquid crystal layer 32 has a retardation Δn·d of 287 nmand the twist angle is set to 45°. Further, it should be noted that theresult of FIG. 85 is for the case in which the positive and negativeretardation films are provided as indicated in FIG. 64 such that thetotal retardation R of the retardation films (33A)₁ and (33B)₁ is set to25 nm and the retardation R′ of the retardation film (33A)₂ and (33B)₂is set to 160 nm.

Referring to FIG. 85, it should be noted that the area of theviewing-angle in which the contrast ratio decreases below 10 issubstantially limited, and the liquid crystal display device showsexcellent viewing-angle characteristics.

FIG. 86 shows the viewing-angle characteristics of the same liquiddisplay device obtained by a simulation. The result of FIG. 66 indicatesthat there is a further possibility that the viewing-anglecharacteristics of the liquid display device be improved by a furtheroptimization.

FIG. 87 shows a construction of a direct-view type liquid crystaldisplay apparatus 130 constructed by using the VA-mode liquid crystaldisplay device of any of the foregoing embodiments.

Referring to FIG. 87, the liquid crystal display apparatus 130 includesa VA-model liquid crystal display device 101, which may be any of theliquid crystal display devices 10-120 explained heretofore, and a planarlight source unit 103 disposed behind the liquid crystal display device101. The liquid crystal display device 101 includes a plurality of pixelregions 102, wherein each of the pixel regions modulates the opticalbeam emitted by the planar light source unit 103. As usual, the planarlight source unit 103 includes a light source part 106 that accommodatestherein a linear light source such as a fluorescent tube and an opticaldiffusion part 104 that causes a diffusion of the light produced by thelinear light source. As a result of such a diffusion, a two-dimensionalillumination of the liquid crystal display device 101 becomes possible.

By using the liquid crystal display device explained heretofore for theliquid crystal display device 101, excellent viewing-anglecharacteristics are obtained, in addition to the high contrast and highresponse representation.

In the VA-mode liquid crystal display device of the present inventiondescribed heretofore, in which a liquid crystal having a negativedielectric anisotropy is used, it is also possible to use a liquidcrystal having a positive dielectric anisotropy (so-called p-type liquidcrystal). As the optimization of the optical properties of the liquidcrystal device described heretofore is not affected by the nature of theliquid crystal whether it is an n-type liquid crystal or a p-type liquidcrystal, the conclusion derived heretofore about the n-type liquidcrystal display device is applicable also to a p-type liquid crystaldisplay device. The only difference is the mode of driving the liquidcrystal device as explained with reference to FIGS. 4A and 4B and FIGS.5A and 5B.

In the embodiment of FIG. 54, 60 or 64, it should be noted that theretardation film (33A)₁ or (33B)₁ should have a very small retardationof 120 nm or less. Such a birefringence film having a very smallretardation is obtained by using a norbornene resin having a norbornenestructure in the principal chain. It should be noted the norborneneresin is almost optically isotropic and can be conveniently used forforming the foregoing retardation films (33A)₁ and (33B)₁.

Thirteenth Embodiment

FIG. 88 shows the construction of a liquid crystal display device 140according to a thirteenth embodiment of the present invention, whereinthose parts corresponding to the parts described heretofore aredesignated by the same reference numerals and the description thereofwill be omitted.

Referring to FIG. 88, the liquid crystal display device 140 has aconstruction similar to that of the liquid crystal display device 40 ofFIG. 54 except that the retardation films (33B)₁ and (33B)₂ are disposedsuch that the retardation axis n_(x) and the retardation axis n_(y)intersect perpendicularly.

FIG. 89 shows the black-mode transmittance Tb of the liquid crystaldisplay device 140 for a case in which the retardation R₂ of theretardation film (33B)₂ is set to 150 nm and the retardation R₁ of theretardation film (33B)₁ is changed variously.

Referring to FIG. 89, it should be noted that black-mode transmittanceTb becomes minimum when the sum of the retardation R₁ and theretardation R₂ is generally equal to the retardation Δn·d of the liquidcrystal layer 32.

FIG. 90 shows the foregoing black-mode transmittance Tb of the liquidcrystal display device 89 for various polar angles for the constructionsof the liquid crystal display device shown in FIGS. 91A-91D.

Referring to FIG. 90, the polar-angle-dependency of the black-modetransmittance Tb, and hence the viewing-angle characteristics of theliquid crystal display device 140, is improved substantially when theretardation film (33B)₁ and the retardation film (33B)₂ are disposedsuch that the retardation axis of the retardation film (33B)₁ adjacentto the liquid crystal layer 32 intersects the absorption axis of thepolarizer 34B as indicated in FIG. 91B or FIG. 91D. In the case of theconstruction of FIG. 91C, on the other hand, it should be noted that theviewing-angle characteristic is deteriorated as compared with the casein which the retardation films are not provided.

FIG. 92A shows the viewing-angle characteristics of the liquid crystaldisplay device 140 in comparison with the viewing-angle characteristicsof FIG. 92B for the case in which the retardation films are notprovided. FIGS. 92A and 92B, it should be noted that the hatched regionindicates the region in which the contrast is smaller than about 1.FIGS. 92A and 92B, it will be understood that the liquid crystal displaydevice 140 shows a superior viewing-angle characteristic to the liquidcrystal display device in which no retardation film is provided.

It should be noted that the characteristic of FIG. 92A is obtained alsoin the case in which a positive liquid crystal having a positivedielectric anisotropy is used for the liquid crystal layer 32.

Fourteenth Embodiment

FIG. 93 shows the construction of a liquid crystal display device 150according to a fourteenth embodiment of the present invention, whereinthose parts corresponding to the parts described previously aredesignated by the same reference numerals and the description thereofwill be omitted.

Referring to FIG. 93, the liquid crystal display device 150 uses asp-type liquid crystal including p-type liquid crystal molecules 32 a forthe liquid crystal layer 32, such that the tilt angle of the liquidcrystal molecules 32 a is controlled in response to the drive voltageapplied across the electrodes 31 a′ and 31 b′. The glass substrates 31Aand 31B are covered by a molecular alignment film (not shown), and themolecular alignment film interacts with the liquid crystal molecules 32a such that the liquid crystal molecules 32 a are aligned generallyperpendicularly to the principal surface of the substrate 31A or 31B inthe non-activated state of the liquid crystal display device 150. In theconstruction of FIG. 93, the liquid crystal display device 150 furtherincludes the positive retardation film (33B)₁ and the negativeretardation film (33B)₂ above the upper glass substrate 31A similarly tothe construction of FIG. 54.

FIG. 94 shows the viewing-angle characteristic of the liquid crystaldisplay device 150 of FIG. 93 for a case in which the positive liquidcrystal ZLI-4792 of E. Merck, Inc. is used for the liquid crystal layer32 and in which the retardation R of the retardation film (33B)₁ and theretardation R′ of the retardation film (33B)₂ are set to 25 nm and 240nm, respectively. In the evaluation of FIG. 94, it should further benoted that the JALS204 film of Nippon Synthetic Rubber, Co., LTD. isused for the molecular alignment film and the thickness of the liquidcrystal layer 32 is set to 3.5 μm.

Referring to FIG. 94, it will be understood that the liquid crystaldisplay device 150 has a viewing-angle characteristic similar to thoseobtained in the previous embodiments such as the embodiment of FIG. 65.

It should be noted that a similar viewing-angle characteristic isobtained also in the liquid crystal display device of FIGS. 5A and 5B.Further, the liquid crystal display device of FIGS. 5A and 5B or theliquid crystal display device of FIG. 93 is easily modified to have anactive matrix construction indicated in FIG. 80. In this case, too, anexcellent view angle characteristic is obtained.

Further, the present invention is not limited to the embodimentsdescribed heretofore, but various variations and modifications may bemade without departing from the scope of the invention.

1. A liquid crystal display device, comprising: first and secondsubstrates sandwiching therebetween a liquid crystal layer; first andsecond electrodes provided respectively on said first substrate and saidsecond substrate; a first polarizer disposed at an outer side of saidfirst substrate away from an inner side thereof facing said liquidcrystal layer; and a second polarizer disposed at an outer side of saidsecond substrate away from an inner side thereof facing said liquidcrystal layer; said liquid crystal layer comprising a liquid crystalhaving a negative dielectric anisotropy in such a manner that moleculesof said liquid crystal are aligned generally perpendicularly to saidfirst and second substrates in a state in which an external electricfield is not applied, said first polarizer and said second polarizerbeing disposed such that an optical absorption axis of said firstpolarizer and an optical absorption axis of said second polarizer crosswith each other perpendicularly, wherein there is disposed an opticallybiaxial retardation film having refractive indices n_(x), n_(y) andn_(z) respectively in x-, y- and z-directions, said refractive indicesn_(x), n_(y) and n_(z) satisfying either of a relationshipn_(x)>n_(y)>n_(z) or n_(y)>n_(x)>n_(z) in at least one of a first gapformed between said first substrate and said first polarizer and asecond gap formed between said second substrate and said secondpolarizer, in such a manner that an in-plane retardation axis of saidoptically biaxial retardation film is oriented either generally parallelor generally perpendicular with said optical absorption axis of saidfirst or second polarizer that is disposed at the same side of saidoptically biaxial retardation film with regard to said liquid crystallayer, wherein said optically biaxial phase retardation film provides anin-plane retardation having a magnitude defined by |n_(x)-n_(y)|·d, drepresenting a thickness of said optically biaxial retardation film, andwherein said in-plane retardation of said optically biaxial retardationfilm has a value of 20 nm or more but not exceeding 60 nm.
 2. The liquidcrystal display device as claimed in claim 1, wherein said opticallybiaxial phase retardation film provides a retardation in a thicknessdirection thereof with a magnitude represented as {(n_(x)+n_(y))/2−n_(z)}·d, d representing a thickness of said optically biaxial retardationfilm, said magnitude not exceeding twice a magnitude of retardation ofsaid liquid crystal layer.
 3. The liquid crystal display device asclaimed in claim 1, wherein said optically biaxial retardation film isprovided in both of said first gap and said second gap.